The present invention relates generally to integrated circuit transistors and in particular the present invention relates to methods of forming damascene transistor gates having a notched profile.
Integrated circuit manufacturers continually strive to scale down semiconductor devices in integrated circuit chips. Smaller scale semiconductor devices translate to increased speed and capacity while reducing power consumption. For example, in order to provide increased capacity in memory chips such as SRAM, it is highly desirable to shrink the size of each memory cell without significantly affecting performance. This may be accomplished by shrinking the size of each component of the memory cell, packing the components closer together, or both.
Integrated circuit transistors have source and drain regions and a gate electrode. The transistors are typically fabricated such that each have a doped polysilicon gate electrode. The source and drain regions are typically implanted into a substrate of silicon. A channel region is defined between the source and drain regions and beneath the gate electrode. A capacitance, known as overlap capacitance, may be created between the gate and the source/drain regions where the gate overlaps the source/drain regions. This capacitance affects how the transistor functions and is undesirable.
Additionally, for high performance devices, such as SRAM, it is desirable to form the shortest channel length transistors at a given lithography node. The channel length is the distance between the source and the drain. However, lithographic processes are limited, and fabrication processes are exploited to form transistors having channel lengths shorter than those possible with lithography alone. One such fabrication process is taught in U.S. Pat. No. 5,834,817 to Satoh et al. Satoh et al. utilizes a plasma etching method and layers having different etching speeds to form shaped gate electrodes. However, this method can present difficulties with control of notch height and depth over a wide process range. Therefore, a need exists for a method of forming notched gate electrodes that allows the notch height and depth to be independently adjusted while providing transistors with shorter channel length and reduced overlap capacitance.
This need is met by the present invention that provides methods for forming notched gate electrodes while allowing independent control of notch height and depth. These methods may be used in conjunction with conventional processing to provide transistors having shorter channel lengths.
In accordance with one embodiment, a method of forming a notched gate is provided. The method comprises: supplying a substrate; forming a dummy gate on the substrate, the dummy gate having sidewalls, and comprising: a first layer proximate to the substrate; a second layer proximate to the first layer; and a third layer proximate to the second layer; etching the second layer to form laterally recessed notches in the dummy gate; forming sidewall spacers on the sidewalls of the dummy gate; depositing a layer of dielectric material over the dummy gate; removing the dielectric material overlying the dummy gate; removing the dummy gate to form a recess between the sidewall spacers; forming a gate oxide in the recess; and depositing a permanent gate material in the recess to form a notched gate. The etching of the second layer may be by a wet etch, and the first and third layer are selected to be resistant to the wet etch. The first layer may comprise a sacrificial gate oxide. The second layer may be silicon nitride, and the third layer may be polysilicon. The wet etch may utilize hot phosphoric acid. The gate material may be selected from a group consisting of a polysilicon, tungsten, tungsten silicide, titanium silicide, cobalt silicide, nickel silicide, molybdenum, titanium, titanium nitride, and aluminum. The dielectric material overlying the dummy gate may be removed by chemical mechanical polishing of the dielectric material.
The method may also comprise forming extension regions in the substrate adjacent to the dummy gate and forming doped regions in the substrate adjacent to the sidewall spacers. The doped regions and the extension regions comprise the source/drain regions. The dummy gate may be formed by: forming the first, second, and third layers; forming a patterned mask over the third layer, etching through the first, second, and third layers to the substrate in areas defined by the patterned mask, and stripping the patterned mask from the third layer. The layer of dielectric material may be formed by depositing an inter-layer dielectric material over the substrate and the dummy gate.
A method of forming a semiconductor device is provided. The method comprises: supplying a substrate; forming a well region in the substrate; forming at least one isolation region the substrate; forming a dummy gate having sidewalls on the substrate over said well region, wherein the dummy gate has a first layer proximate to the well region, a second layer proximate to the first layer; and a third layer proximate to the second layer; etching the second layer to form laterally recessed notches in the sidewalls of the dummy gate; forming sidewall spacers on the sidewalls of the dummy gate; depositing a layer of dielectric material over the dummy gate; removing the dielectric material overlying the dummy gate; removing the dummy gate to form a recess between the sidewall spacers; forming a gate oxide in the recess; and depositing a permanent gate material in the recess to form a notched gate. The etching of the second layer may be by a wet etch, and the first and third layer are selected to be resistant to the wet etch. The first layer may comprise a sacrificial gate oxide. The second layer may be silicon nitride, and the third layer may be polysilicon. The wet etch may utilize phosphoric acid. The gate material may be selected from a group consisting of a polysilicon, tungsten, tungsten silicide, titanium silicide, cobalt silicide, nickel silicide, molybdenum, titanium, titanium nitride, and aluminum. The isolation region may be an isolation trench.
The method may further comprise forming extension regions in the well region adjacent to the dummy gate and forming doped regions in the well region adjacent to the sidewall spacers. The doped regions and the extension regions comprise the source/drain regions. The dummy gate may be formed by: forming the first, second, and third layers; forming a patterned mask over the third layer, etching through the first, second, and third layers to the well region in areas defined by the patterned mask, and stripping the patterned mask from the third layer. A pocket implant may be formed under the extension regions. The isolation region may be an isolation trench. The layer of dielectric material may be formed by depositing an inter-layer dielectric material over the dummy gate and the substrate.
In accordance with another embodiment, a method of controlling the notch dimensions of a damascene notched gate is provided. The method comprises: supplying a substrate; forming a dummy gate having sidewalls on the substrate, wherein the dummy gate comprises a first layer proximate to the substrate, a second layer having a selected thickness proximate to the first layer, and a third layer proximate to the second layer; etching the second layer to form laterally recessed notches in the dummy gate, wherein the etch is controlled to select the depth of the laterally recessed notches and the height of the notches is determined by the selected thickness of the second layer; forming sidewall spacers on the sidewalls of the dummy gate; depositing a of dielectric material over the dummy gate; removing the dielectric material overlying the dummy gate; removing the dummy gate to form a recess between the sidewall spacers; forming a gate oxide in the recess; and depositing a permanent gate material in the recess to form a notched gate. The height may be in the range of about 100-500 xc3x85, and the depth may be in the range of about 50-200 xc3x85. The etching of the second layer may be by a wet etch, and the first and third layer are selected to be resistant to the wet etch. The first layer may comprise a sacrificial gate oxide. The second layer may be silicon nitride, and the third layer may be polysilicon. The wet etch may utilize phosphoric acid.